1. Field of the Invention
The present invention generally relates to a contactless IC card system. More specifically, the present invention is directed to an amplifying circuit, a modulating circuit, a demodulating circuit, a transmitter apparatus, and a receiver apparatus, applicable to a contactless IC card capable of reading/writing various sorts of data in a contactless manner, and is also applicable to an IC card reader/writer capable of communicating data with this contactless IC card.
2. Description of the Related Art
Conventionally, in IC card systems which employ of IC cards, these IC card systems are applied to ticket inspection systems used in transportation facilities, and person entrance/exit management systems for rooms. Such conventional IC card systems utilize IC cards carried by users and IC card readers/writers capable of transmitting/receiving various sorts of data between the IC cards and the readers/writers. These data may be transmitted/received in the contactless manner between the IC cards and the IC card readers/writers.
In other words, in this sort of IC card system, an IC card reader/writer modulates a carrier wave having a preselected frequency by using a desired data stream so as to produce a transmission signal, and then transmits this produced transmission signal to the IC card.
The IC card receives this transmission signal via an antenna, and then demodulates this transmission signal to decode the data sent from the IC card reader/writer. Furthermore, the IC card modulates internally saved data, such as personal data, by using a preselected carrier wave in response to this received data, and then sends out the modulated data to the IC card reader/writer.
Then, the IC card reader/writer receives the data sent from this IC card. Based upon this received data, a door of a ticket inspection machine is opened/closed. Also, any person is allowed to enter into a room, and/or to come out from this room.
In such a conventional IC card system, these data are modulated by the ASK (Amplitude Shift Keying) modulating method, and then, the ASK-modulated data is transmitted/received between the IC card and the card reader/writer. Conventionally, as such a modulating means for an ASK modulation signal, a modulating circuit with using a variable gain amplifying circuit, and a multiplying circuit is employed. Also, as such a demodulating means for the ASK modulation signal, a demodulating circuit with using an envelope detecting circuit constructed of a diode, and using a synchronization detecting circuit is employed.
FIG. 1 is a schematic block diagram for showing one conventional modulating circuit constructed of this variable gain amplifying circuit. In this modulating circuit 1, the gain of the variable gain amplifying circuit 2 is switched in response to a logic level of a data stream D to be sent. Also, a carrier signal SC is amplified by this variable gain amplifying circuit 2. As a result, this modulating circuit 1 modulates the amplitude of the carrier signal SC outputted from the variable gain amplifying circuit 2 in response to the logic level of the data stream D so as to produce an ASK modulation signal SM.
Also, FIG. 2 is a schematic block diagram for indicating another conventional modulating circuit arranged by a balanced modulating circuit with employment of a multiplying circuit. In this modulating circuit 3, the carrier signal SC is multiplied by the data stream D in the multiplying circuit 4, and while the amplitude of this carrier signal SC is varied in response to the logic level of the data stream D, the ASK modulation signal SM is produced.
In contrast, FIG. 3 is a schematic block diagram for showing one conventional demodulating circuit arranged by an envelope detecting circuit with employment of a diode. In this demodulating circuit 6, the ASK modulation signal SM is rectified by employing the diode D. Furthermore, this rectified ASK modulation signal SM is entered into a smoothing circuit having a predetermined time constant defined by a resistor R and a capacitor C. As a result, the envelope-detected output of the ASK modulation signal is outputted as the demodulation signal SD.
FIG. 4 is a schematic block diagram for representing another conventional demodulating circuit arranged by a synchronization detecting circuit. In this demodulating circuit 8, a carrier signal component SCC is extracted from the ASK modulation signal SM by employing a phase-synchronization system circuit 9 arranged by, for example, a filter circuit arrangement and a PLL circuit arrangement. Both this carrier signal component SCC is multiplied by the ASK modulation signal SM by a multiplying circuit 10. In the demodulating circuit 8, a baseband component is extracted from the multiplied result of this multiplying circuit 10 by a low-pass filter (LPF) 12 to thereby be outputted as the demodulation signal SD.
On the other hand, the following demands are made in these conventional IC card systems. That is, these modulating circuits and demodulating circuits can be simply and readily manufactured in the IC form in combination with other circuit blocks. Moreover, these modulating/demodulating circuits can be operated in high efficiencies.
Moreover, these modulating circuits and demodulating circuits with employment of the conventional circuit arrangements can hardly satisfy the necessary items for the IC card systems.
In further detail, in the modulating circuit arranged by the variable gain amplifying circuit, the voltage range which can be effectively utilized by the variable gain amplifying circuit is limited. This voltage range limitation causes the lower power efficiency of the conventional modulating circuit. Also, as to the modulating circuit with employment of the multiplying circuit, there are such drawbacks that the circuit arrangement becomes complex, and this complex modulating circuit cannot be simply and readily manufactured in the IC form.
For instance, also in the power amplifying circuit for the ASK modulation signal, which is similarly required to be manufactured in the IC form similar to such modulating circuits, the ASK modulation signal must be amplified while saving a change contained in the amplitudes of this ASK modulation signal. After all, this power amplifying circuit must be operated in a better linearity region. This causes the power efficiency to be lowered also in the power amplifying circuit. Also, this power amplifying circuit has a drawback in that in order to transmit sufficiently high power, the active elements capable of satisfying the necessary allowable current and the allowable loss must be used instead of commercially available general-purpose electronic components.
In contrast, the demodulating circuit arranged by the envelope detecting circuit with employment of the diode owns such a drawback that a leakage current is produced in the diode when this demodulating circuit is manufactured in the IC form, and therefore, the detection efficiency of the ASK modulation signal is considerably lowered. In other words, as shown in FIG. 5, when an envelope detecting circuit with employment of a diode is manufactured in the IC form, both polarities must be set to floating potentials at the diode D. As a result, a stray transistor is necessarily produced. Accordingly, as represented in FIG. 6, a leakage current is produced.
On the other hand, in the conventional demodulating circuit arranged by the synchronization detecting circuit, there is another drawback in that the circuit arrangement of the phase synchronization system circuit 9 becomes complex.